Recently, as for information instruments, ones equipped with a touch panel display have been becoming mainstream, as seen in tablet PCs, smartphones, e-book readers, etc. A touch panel display has a structure where a touch sensor glass and a cover glass are layered on a glass substrate for display. There is also known an integrated configuration of a touch sensor glass and a cover glass, which is called OGS (one glass solution).
In a touch sensor glass, a cover glass and an OGS glass, any glass is desired to be thin and have a high strength, for which a glass that has been chemically strengthened through ion exchange, that is, a chemically strengthened glass is used.
The strength characteristics of these chemically strengthened glasses are generally expressed as a surface compressive stress (CS, compressive stress) and a depth of compressive stress (DOL, depth of layer). In the case where a raw sheet glass of ordinary soda lime glass is subjected to chemical strengthening treatment, in general, a chemically strengthened glass having CS of 500 to 600 MPa and DOL of 6 to 10 μm can be obtained.
There has been proposed an aluminosilicate glass having an easily ion-exchangeable composition for enhancing the strength, and in the case where a raw sheet glass of an aluminosilicate glass is subjected to the same chemical strengthening treatment, there can be obtained a chemically strengthened glass having CS of 700 to 850 MPa and DOL of 20 to 100 μm.
These glasses for chemical strengthening are produced according to a float process or a fusion process (also referred to as an overflow downdraw process). The float process is known as production method for windowpanes in buildings and the like, and this is a method of casting a molten glass onto a molten tin and forming it into a planar form. The other fusion process is known as a production method for alkali-free glasses for displays and the like, and this is a method of overflowing a glass down to both sides from an upper gutter and fusing it at the tip of the lower sword to form it into a planar form. For the glass for chemical strengthening, in general, a soda lime glass is produced according to a float process, and an aluminosilicate glass is produced according to both production methods of a float process and a fusion process.
Soda lime glass produced according to a float process is inexpensive as compared with aluminosilicate glass. However, regarding the chemically strengthened glass of an already-existing soda lime glass, CS thereof could hardly be increased to a level of glass strengthening that has become required recently. Accordingly, there has been proposed a chemical strengthening treatment method that can increase the glass strength in a chemically strengthened glass using a soda lime glass (for example, see PTL 1).